: This project is aimed at developing a system in which cloned odor receptors from the mammalian olfactory system can be expressed at a high level and assayed for odor binding activity. Previous attempts to accomplish this in heterologous expression systems have failed, apparently due to missing elements required either for proper expression or coupling to the second messenger cascade required for signaling. To circumvent those difficulties this project will use olfactory neurons themselves as the expression system for odor receptors. Gene transfer will be accomplished by using an adenovirus vector to infect the nasal epithelium and transfer the recombinant odor receptor gene to normal olfactory neurons. Upon expression of the recombinant receptor infected neurons will express 2 odor receptors-the normal one and the cloned receptor. Since any particular endogenous odor receptor is only expressed in about 0.02% of the olfactory neurons and the adenovirus infects at least 25% of the neurons, the exogenous cloned receptor will be expressed by at least 100 times as many cells as any normal receptor. To assay for odor receptor expression the EOG will be used to measure changes in electrical potentials in response to application of a battery of odors. In this way a particular receptor can be matched to the odor(s) that best activate it. This will provide several important lines of evidence that are critical to understanding the olfactory system's ability to discriminate among an enormous odor repertoire. First it will provide the first direct evidence that the large family of G-protein coupled receptors cloned from nasal tissue are in fact the odor receptors. Second it will enable us to develop a "pharmacology" of odor receptors, as has been done for other G-protein coupled receptors. This will answer critical questions concerning the relation between amino acid sequence, protein structure and receptor affinity. Are receptors that are closely related in nucleotide or amino acid sequence also sensitive to similar odor compounds?